Box-Line Reduction (Claiming) in Sudoku
When all instances of a candidate in a row or column are contained within a single 3×3 box, that row/column "claims" the digit inside that box, eliminating it from all other cells in the box.
Interactive Box-Line Reduction Trainer (Row 2 → Box 1)
In Row 2, candidate 7 only appears in R2C2 and R2C3 (inside Box 1). Click Solve to eliminate 7 from the rest of Box 1 (R3C1 and R3C2).
3-Step Identification Process
Scan Rows & Columns
Examine a row or column and locate a candidate number that only appears in 2 or 3 cells.
Check Box Containment
Determine if all candidate instances in that row or column reside within the SAME 3×3 box.
Prune the Rest of Box
Eliminate that candidate from all other unplaced cells inside that 3×3 box.
Frequently Asked Questions
What is Box-Line Reduction in Sudoku?
Box-Line Reduction (also called Locked Candidates Type 2 or Claiming) occurs when a candidate digit in a specific row or column is confined entirely within a single 3×3 box. Because that row or column must place its digit within that box, no other cell in that same 3×3 box can contain that digit.
What is the difference between Pointing Pairs and Box-Line Reduction?
Pointing Pairs starts from the Box (candidates in a box lock into a line, eliminating candidates from the rest of the line). Box-Line Reduction starts from the Line (candidates in a line are confined to one box, eliminating candidates from the rest of the box).
When should I look for Box-Line Reduction?
Look for Box-Line Reduction whenever you are solving Hard or Expert Sudoku puzzles after filling in full pencil marks. It is particularly helpful when rows or columns have only 2 or 3 open spots for a specific number.
Ready to try on a live puzzle?
Put your Locked Candidates deduction skills to work on Hard Sudoku puzzles.